Supernovae ought to happen in our galaxy about once every 30 years. Until 1987, however, no bright supernova had been seen since 1604. By no means were we "due for one"—that is an all-too-frequent abuse of statistics—but nevertheless a supernova did explode on February 23, 1987. (To be precise, that was the date that light from the explosion first reached us; the actual explosion took place 170,000 years earlier.) It was not in our galaxy, but in one of the small satellite galaxies orbiting it, the Large Magellanic Cloud, visible in the southern hemisphere. It was a distance of 170,000 light-years from Earth, and became known as SN 1987A. The star that exploded was a blue supergiant, probably 20 times more massive than the Sun, and when it exploded it was visible to the naked eye. Supernova 1987A became one of the most studied events in astronomical history, as observations of the expanding blast revealed numerous exciting results.
As a star's core collapses, the protons and electrons in it are smashed together to form a neutron, and every time such a reaction happens, an evanescent particle called a neutrino is created. The neutrinos travel outward from the core at the speed of light, and they are created in such vast numbers that they carry off most of the energy produced during the supernova. Just before SN 1987A became visible, surges of neutrinos were indeed detected here on Earth. Since the neutrinos escape from the star before the visible explosion, the timing of the event provided important observational support for the idea that core collapse and the subsequent bounce of infalling outer layers drives some supernova explosions.
There are several stars in Earth's celestial neighborhood that are prime supernova candidates. Betelgeuse, the red supergiant that marks Orion's right shoulder, and Antares, the brightest star in Scorpio, are two notable examples.
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Jeffrey C. Hall